The present invention relates to an air conditioner having a refrigerant circuit. More particularly, the present invention pertains to a displacement control valve used in a variable displacement compressor in a refrigerant circuit.
A typical refrigerant circuit of a vehicle air conditioner includes a condenser, an expansion valve, an evaporator and a compressor. The compressor receives refrigerant gas from the evaporator. The compressor then compresses the gas and discharges the gas to the condenser. The evaporator transfers heat to the refrigerant in the refrigerant circuit from the air in the passenger compartment. The pressure of refrigerant gas at the outlet of the evaporator, in other words, the pressure of refrigerant gas that is drawn into the compressor (suction pressure Ps), represents the thermal load on the refrigerant circuit.
Variable displacement swash plate type compressors are widely used in vehicles. Such compressors include a displacement control valve that operates to maintain the suction pressure Ps at a predetermined target level (target suction pressure). The control valve changes the inclination angle of the swash plate in accordance with the suction pressure Ps for controlling the displacement of the compressor. The control valve includes a valve body and a pressure sensing member such as a bellows or a diaphragm. The pressure sensing member moves the valve body in accordance with the suction pressure Ps, which adjusts the pressure in a crank chamber. The inclination of the swash plate is adjusted, accordingly.
In addition to the above structure, some control valves include an electromagnetic actuator, such as a solenoid, to change the target suction pressure. An electromagnetic actuator urges a pressure sensing member or a valve body in one direction by a force that corresponds to the value of an externally supplied current. The magnitude of the force determines the target suction pressure. Varying the target suction pressure permits the air conditioning to be finely controlled.
Such compressors are usually driven by vehicle engines. Among the auxiliary devices of a vehicle, the compressor consumes the most engine power and is therefore a great load on the engine. When the load on the engine is great, for example, when the vehicle is accelerating or moving uphill, all available engine power needs to be used for moving the vehicle. Under such conditions, to reduce the engine load, the compressor displacement is minimized. This will be referred to as a displacement limiting control procedure. A compressor having a control valve that changes a target suction pressure raises the target suction pressure when executing the displacement limiting control procedure. Then, the compressor displacement is decreased such that the actual suction pressure Ps is increased to approach the target suction pressure.
The graph of FIG. 17 illustrates the relationship between suction pressure Ps and displacement Vc of a compressor. The relationship is represented by multiple lines in accordance with the thermal load in an evaporator. Thus, if the suction pressure Ps is constant, the compressor displacement Vc increases as the thermal load increases. If a level Ps1 is set as a target suction pressure, the actual displacement Vc varies in a certain range (xcex94Vc in FIG. 17) due to the thermal load. If a high thermal load is applied to the evaporator during the displacement limiting control procedure, an increase of the target suction pressure does not lower the compressor displacement Vc to a level that sufficiently reduces the engine load.
Thus, the compressor displacement is not always controlled as desired as long as the displacement is controlled based on the suction pressure Ps.
Accordingly, it is an objective of the present invention to provide an air conditioner and a control valve used in a variable displacement compressor that accurately control the compressor displacement regardless of the thermal load on an evaporator.
To achieve the above objective, the present invention provides an air conditioner including a refrigerant circuit. The refrigerant circuit has a condenser, a decompression device, an evaporator and a variable displacement compressor. The compressor has a discharge pressure zone, the pressure of which is a discharge pressure, and a suction pressure zone, the pressure of which is a suction pressure. The refrigerant circuit further has a high pressure passage extending from the discharge pressure zone to the condenser and a low pressure passage extending from the evaporator to the suction pressure zone. A displacement control mechanism controls the displacement of the compressor based on the pressure difference between the pressure at a first pressure monitoring point located in the refrigerant circuit and the pressure at a second pressure monitoring point located in the refrigerant circuit. The first pressure monitoring point is located in a section of the refrigerant circuit that includes the discharge pressure zone, the condenser and the high pressure passage. The second pressure monitoring point is located in a section of the refrigerant circuit that includes the evaporator, the suction pressure zone and the low pressure passage.
The present invention also provides a control valve for controlling the pressure in a crank chamber of a compressor to change the displacement of the compressor. The compressor has a discharge pressure zone, the pressure of which is a discharge pressure, a suction pressure zone, the pressure of which is a suction pressure, and an internal gas passage that includes the discharge pressure zone, the crank chamber and the suction pressure zone. The control valve comprises a valve housing, a valve body, a pressure receiver and an actuator. The valve body is located in the valve housing to adjust the size of an opening in the internal gas passage. The pressure receiver actuates the valve body in accordance with the pressure difference between the discharge pressure and the suction pressure thereby causing the pressure difference to seek a predetermined target value. The actuator urges the valve body by a force, the magnitude of which corresponds to an external command. The urging force of the actuator represents the target value of the pressure difference.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.